This invention relates to methods for isotope enrichment.
Selected isotopes are useful in a variety of applications. For example, it is often desired to have an isotopic marker present in a molecule to trace such molecule in biologic processes. Radioactive isotopes are used for curative purposes in cancer and for medical radiology. Isotopic enrichment is used by industry to control physical properties of materials such as thermal conductivity. Separation of isotopes is conventionally accomplished by a variety of means relying on difference in mass of isotopes. One example of this utilizes a gaseous diffusion process to separate heavier components of an isotopic mixture from the lighter components by reason of their differing mass. Physical separation by fractionation to isolate a particular desired isotope is also known. Conventionally, the most popular procedure for isotope enrichment is the gaseous-diffusion method, which depends on the relative rates of gaseous diffusion through molecular barriers by different isotopes of a given element. Such processes typically operate at reduced pressure and elevated temperature. Often the difference between the mass of the isotopes is very small and although the lighter weight isotope diffuses more rapidly through porous barriers, the separation by each barrier is very, very small, therefore, many or a cascade of barriers are required.
More recently, there has been an interest in attempting to cause isotope separation by multiple photon absorption and photoionization. These processes are labor intensive and difficult to accomplish. Typically, several iterations are required to achieve the high concentration of the desired isotopic component in the separated product. An example of separation by multiple photon absorption can be found in U.S. Pat. No. 4,655,890 where a mixture of molecules is irradiated with infrared laser radiation having a first wavelength which lies in close coincidence with the transition from the ground rotation-vibration level of the molecules which contain the isotopic variant of the element which is to be separated, and the next highest rotation-vibration level having vibrational excitation. This radiation selectively induces multiple photon absorption by those molecules. Then it is possible to cause the selectively excited molecules to undergo chemical reaction while other isotopic variance of the element remains substantially in their lower rotation-vibration levels and consequently unreactive, thereby causing a product of the chemical reaction of the selectively excited molecules to be produced and separable from the unreacted molecules.
In the parent of the present application, a novel process and system apparatus were described for separating isotopes of an element and causing enrichment of a desired isotope of an element in a material. In one aspect, a laser was used to modify or fabricate a material so as to produce a desired isotopic content in the material, which differs from that which naturally occurs.
Isotope enrichment in laser ablation plumes is predominantly an ionic process. Such a process involves the interaction of energetic ions produced in the ablation plasma with self generated magnetic fields in the same plasma. Details about the theory of this self generated magnetic field process and its effects on the motion of isotopic ions in an ablation plasma continue to be studied and worked out. However, it is recognized that by maximizing the ionic component of the plasma, the number density of isotopes that undergo the enrichment process can be increased.
Referring to FIGS. 19A and 19B, it can be seen that the ratio of ions to neutrals in a single ablation plasma plume is relatively small. Typically that ratio is on the order of 1% or less. To achieve a practical method for using this phenomenon as a way to harvest enriched isotopes, it would be desired to increase this ionic component to much greater levels.
A single laser ablation plume is the result of a number of relatively complex processes occurring during the absorption of a laser pulse by a material and subsequent expansion of the gaseous/plasma plume. One part of this process involves the initial production of a very large percentage of ions if not complete ion content in the vapor phase plasma. In other words, the system starts out as a fully ionized electron-ion plasma. Because of the high initial density and ensuing collisions in at the early vapor/plasma stage, however, a great deal of electronic neutralization occurs as the plasma expands away from the surface of the ablating material. This highly neutralized plasma then moves through the region of the self-generated magnetic field and the ionic component undergoes the isotope enrichment.
A plasma having a density which exceeds a certain level, known as the xe2x80x9ccritical densityxe2x80x9d, begins to reflect optical radiation rather than absorb it. The absorption properties of a plasma normally increase with the density of free electrons in it. However, when the density reaches the critical density the plasma becomes nearly totally reflective. This limits the amount of internal energy that can be added to a plasma by a laser pulse. In most cases where an ablation plasma is formed by a laser this critical density is achieved in the early part of the laser pulse impinging on the surface of the material.
The present invention provides a novel process and system apparatus for separating isotopes of an element and causing enrichment of a desired isotope of an element in a material. In one aspect, the invention utilizes lasers to modify or fabricate a material so as to produce a desired isotopic content in the material, which differs from that which naturally occurs. The reference to a condition different from that which naturally occurs refers to both enhancement and depletion.
In a preferred embodiment, the present invention relies upon laser induced formation of a plasma by means of a pulsed laser beam. The material containing the isotopes desired to be separated is placed in the beam path and is called a target. A plasma is formed from the target material by focusing an intense, short duration optical pulse from the laser onto the target. The concentrated energy contained in the focused and/or concentrated laser beam ionizes the target material, energizing its electrons and raising it to a temperature or otherwise exciting it to a condition whereby ions are produced, thereby generating a plasma. The plasma contains ions having varying isotopes, ion energy and charge state distributions. In one aspect, the ions are contained in a plasma plume generated when laser pulses are directed to a target. The process and system of the invention produce an unusual isotopic enhancement effect in the observed ion spectra. This effect is manifested as an enrichment of the lighter isotope in zones of the ablation plume, as observed normal to the surface of the target material.
In one aspect, the method comprises directing the laser beam to the target at an intensity and wavelength sufficient to generate the plasma comprising ionized isotopic species and to generate an internal electromagnetic scattering field within the plasma causing spatial separation of the ionized isotopic species. In another aspect, the ions are collected on a substrate in a manner which provides zones having isotopic distribution different from that which naturally occurs for a given element contained in the target. It is preferred that the pulses which generate the plume be produced by an ultrafast laser (femtosecond or picosecond). It is preferred that the laser generates the plume by ablation or laser induced breakdown (LIB) of the target material.
In another aspect, the method comprises directing an initial pulse of the laser beam to the target at an intensity and wavelength sufficient to generate the plasma comprising ionized isotopic species and to generate an internal electromagnetic scattering field within the plasma causing spatial separation of the ionized isotopic species. The dense plasma is then allowed to expand so that its volume increases and its electron density decreases. This brings the density of the plasma equal to or less than the critical density. During the expansion process a certain degree of neutralization occurs in the plasma which additionally reduces the free electron concentration. After allowing the expansion to occur, a time delayed second pulse of the laser beam is directed to the plasma to further spatially separate the ionized isotopic species. By correctly delaying the time of the second pulse, additional energy is absorbed by the plasma and the ionic component increases.
The invention provides substantial advantages over conventional methods for separating isotopic components of an element. The invention further provides the ability to effectively enhance the isotopic content of a material as compared to the natural state. Products having desired isotope distribution are able to be produced from the process. The process and system are adaptable to commercial use and automated production. Therefore, isotope enriched products are obtainable by the methods and system of the invention.
Objects, features, and advantages of the invention include, in addition to the foregoing, an improved method and system for separating isotopes of an element, and particularly for forming product material having a desired isotopic content different from that which naturally occurs.
Another object is to provide a method for producing spatial isotopic separation.
Another object is to provide a method and system which utilize lasers to achieve isotope separation or enrichment at certain ablation or scattering angles.
Another object is to provide a method and system which utilize lasers to achieve isotopic separation or isotopic enrichment of a product.
Another object is to provide a method and system for producing products having a desired isotopic distribution.
Another object is to provide a method and system which effectively and economically achieve isotopic separation and isotopic enhancement in a product material.
Another object is to provide a method and system which applies the technique of isotopic separation to chemical element separation.
These and other objects, features, and advantages will become apparent from the following description of the preferred embodiments and accompanying drawings.